1. Field of the Invention
The invention relates to a switching power supply circuit and a control method therefor that generates from an input voltage an output voltage whose target value is a target voltage by driving a switching element at a duty ratio commensurate with the target voltage.
2. Description of the Related Art
As a related-art technology, there exists an electric power supply device that receives a DC input voltage through an input terminal and outputs a lowered DC output voltage from an output terminal (see, e.g., Japanese Patent Application Publication No. 2007-159176 (JP-A-2007-159176)). This electric power supply device has: a pulse width modulation oscillator that performs pulse width modulation (PWM) of an output of an error amplifier; a drive circuit that generates a drive signal from a pulse signal received from the pulse width modulation oscillator; and a pair of power semiconductor switching elements that generates a DC output voltage by lowering the DC input voltage on the basis of the drive signal from the drive circuit.
By the way, in a switching power supply circuit as shown in FIG. 1 which controls an output voltage VTL to a target voltage by switching an input voltage VB, when the target voltage of the output voltage VTL and the input voltage VB are close to each other and therefore the input voltage VB and the output voltage VTL are substantially equal, there sometimes occur alternate repetitions of a continuous on-state period during which an on-state of a switching element Q1 continues and a switching period during which the on-off switching of the switching element Q1 at certain duty ratio continues. This repetition phenomenon occurs periodically as shown in FIG. 2, so that harmonic noise whose fundamental wave has a repetition period T (frequency 1/T) occurs. The repetition frequency VT is determined by, for example, a delay time of a feedback loop that includes an error amplifier (which time includes a phase compensation factor for preventing oscillation), or the like, and sometimes becomes several hundred kHz.
A reason for the occurrence of such a repetition phenomenon is that due to the characteristics of electronic component parts used, the circuit delay times, etc., there is a limit to the minimum pulse width of the PWM signal (duty signal) that the PWM drive circuit outputs.
For example, in the case where a voltage slightly lower than the input voltage VB is set as a target voltage of the output voltage VTL, the actual duty ratio that can be output at the time of the minimum pulse width is about 99% where the theoretical duty ratio (duty value) of the PWM signal for driving the switching element Q1 is, for example, 99.999%. Therefore, when the switching element Q1 is PWM-driven at a duty ratio of 99%, the target voltage is greater than the output voltage VTL (target voltage>output voltage VTL). Hence, if the PWM drive circuit is operated to further increase the duty ratio, the duty ratio is fixed at 100%, so that the switching element Q1 reaches a complete on-state. However, if the switching element Q1 reaches the complete on-state, the target voltage is less than the output voltage VTL (target value<output voltage VTL). Therefore, the PWM drive circuit operates so as to lower the duty ratio of the PWM signal. Hence, the state of operation of the switching element Q1 becomes a state of alternate repetitions of a period during which the switching element Q1 is continuously on and a period during which the switching element Q1 switches on and off at a certain duty ratio.
Besides, when the voltage difference between the target voltage of the output voltage VTL and the input voltage VB becomes very small due to fluctuations of the input voltage VB, the two states, that is, the continuous on-state and the switching state, are sometimes alternately repeated as well.
In some cases, the repeated actions induce a current ripple in a wiring (harness), such as an input wiring of the input voltage VB, an output wiring of the output voltage VTL, etc.